High performance oxygen and fuel electrode for reversible solid oxide fuel cell applications

ABSTRACT

Novel mixed-conducting perovskite oxides, including La0.3Ca0.7Fe0.7Cr0.3O3-δ, useful as oxygen and fuel electrodes for solid oxide fuel cells (SOFCs) and reversible solid oxide fuel cells (RSOFCs) applications. Electrode materials produce by microwave-assisted processes show improved properties as electroactive materials. SOFC and RSOFC are successfully prepared using microwave-assisted techniques.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application62/167,532, filed May 28, 2015, which is incorporated by referenceherein in its entirety.

BACKGROUND OF THE INVENTION

Solid oxide fuel cells (SOFCs) are electrochemical devices that canconvert chemical energy into electrical energy with very highefficiency. SOFCs also have several other advantages overcombustion-based technologies, such as fuel flexibility (H₂,hydrocarbon-based fuels such as CH₄, CO, etc.), low emission ofpollutants (SO_(x) and NO_(x)), and serving to capture CO₂ from theanode exhaust stream in high purity form, already separated from N₂.

A typical SOFC consists of a dense electrolyte and two porouselectrodes, the anode and the cathode. As part of the efforts to developnew energy conversion systems, there is great interest in reversiblefuel cells, particularly reversible solid oxide fuel cells (RSOFCs).RSOFCs are single-unit, all-solid-state, electrochemical devices thatcan operate in both the fuel cell (SOFC) and electrolysis (SOEC) mode,thus acting as flexible energy conversion and storage systems,particularly to store intermittent renewable energy, such as wind orsolar. In the SOFC mode, various fuels, such as H₂, natural gas,hydrocarbons or syngas, are converted spontaneously with oxygen (e.g.,air) at the cathode to electricity and heat. However, when excesselectricity is available, the device can be run in the SOEC mode toconvert the electrical energy back to chemical energy by theelectrolysis of various feedstocks, such as H₂O, CO₂ or CO₂+H₂O to fuel.

The most common degradation and cell failure issue for RSOFCs arises atthe oxygen electrode when the cell is operating in the electrolysis mode(oxygen evolution at the oxygen/air electrode). This is due todelamination of the electrocatalytic material from the electrolyte. Thedelamination mechanism is not fully understood, but several processeshave been postulated, including high oxygen pressure development,morphological changes in air electrodes, and electrolyte grain boundaryseparation [1-5]. At the fuel electrode, key problems are coking,sulphur poisoning and morphological changes leading to performance loss.

Therefore, there is a need in the art for the development of a mixedconducting oxide (MIEC) that can withstand electrolysis conditionswithout delamination, while also exhibiting superior oxygen evolutionand reduction activities. There is further need in the art for mixedconducting oxides for use at the fuel electrode that exhibit resistanceto coking, which retain activity in the presence of sulphur (e.g., H₂S)and which exhibit good retention of performance during operation.Additionally, for implementation of RSOFCs, there is a need for MIECwhich function efficiently in both the fuel cell (SOFC) and electrolysis(SOEC) mode.

To date, the most common materials used in RSOFCs are essentially thesame as those used for SOFC, namely yttria stabilized zirconia (YSZ) asthe electrolyte, a Ni—YSZ cermet as the fuel electrode, and a La_(1-x)Sr_(x)MnO₃ (LSM)-YSZ composite as the air electrode. The search forhigher performance electrode and electrolyte materials for RSOFCs hasbeen a focus of research in recent years, with a particular emphasis onthe development of new air and fuel electrodes. At the air electrode,this has included the development of mixed ionic-electronic conductors(MIECs), such as Fe-based perovskites e.g., SrFeO_(3-δ), and the use ofa variety of cation dopants in both the A and B-sites [6-9]. As anexample, LaCrO₃ and its doped variants are good candidates forapplication ascathode materials in SOFCs [10]. Other high performanceair electrode materials include La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O_(3-δ)(LSCF), which has exhibited a low polarization resistance (Rp) of 0.18Ωcm² at 800° C. [11], La_(0.6)Sr_(0.4)Fe_(0.8)Cu_(0.2)O_(3-δ) (LSFCu),which has demonstrated a very low Rp of 0.07 Ωcm² [12]. An example of agood performing fuel electrode is La_(0.8)Sr_(0.2)Cr_(0.5)Mn_(0.5) O₃(LSCM) (2), which has exhibited a polarization resistance of 0.3 Ωcm² inH₂ at 800° C. [13].

Recently, Chen et al. [14] have shown very good catalytic activity forboth H₂/CO oxidation and O₂ reduction using the same MIEC material atboth electrodes (symmetric electrodes), i.e.,La_(0.3)Sr_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ) (LSFCr), which was used for thefirst time as an SOFC electrode. The selected stoichiometry of thematerial was based on increasing the electronic and ionic conductivityof a Fe-based perovskite by heavy A-site substitution of La by Sr. Inaddition, partial substitution of Fe at the B site by Cr was done tostabilize the orthorhombic perovskite and its associated high level ofvacancy disorder [15].

Herein the performance of derivatives of LSFCr containing calcium, i.e.,La_(0.3)Ca_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ) (LCFCr), are examined in SOFC.In particular, the use of these MIECs as the oxygen and fuel electrodesin RSOFC is examined Additionally, the resistance of the electrodematerials to sulfur is examined.

Usually, MIECs are synthesized by solid-state reactions, where theprocess involves multiple heating (≥1200° C.) and regrinding steps tohelp overcome the solid-state diffusion barrier [16]. Some of thetraditional methods by which MIECs have been prepared include thesol-gel method [6], the EDTA citrate complexing process [12], theauto-ignition process [7], the Pechini method [9], and most commonly, byusing combustion methods [14].

The use of microwave (MW) assisted methods in ceramic materialsprocessing has recently become an active area of research, primarily astheir properties depend so strongly on the fabrication method employed[46, 69, 70]. MW methods have been shown to enhance the rate ofdiffusion of ions and atoms in solid-solid reactions by several ordersof magnitude, thus shortening reaction times and lowering the reactiontemperature [46, 71]. Furthermore, it may be possible to induceinteresting changes in particle morphology and sizes using microwavemethods (87).

MW-assisted techniques are understood to be environmentally friendly[52] as they require less energy than conventional material processingmethods. It is also known that MW sintering of ceramics leads to a morerapid heating rate and a higher efficiency of heating, also resulting ina lower thermal stress gradient due to the local heating of microwaves[53, 72]. The use of MW-assisted processing is relatively new in thedomain of SOFCs.

The main features that distinguish microwave synthesis from conventionalmethods are faster energy transfer rates, i.e., more rapid heatingrates, and the selective heating of materials. This leads to a uniquetemperature distribution within the material when it is heated in amicrowave furnace. During conventional heat treatment, energy istransferred to a material through thermal conduction and convection,creating thermal gradients. However, in the case of microwave heating,energy is transferred directly to the material through an interaction ofthe material at the molecular level with the electromagnetic waves [50].The most important contribution in microwave heating may be that thedipoles in the material follow the alternating electromagnetic fieldassociated with the microwave, with its rapidly changing electric field(ca. 2.4×10⁹ times per second). The resistance to this movementgenerates a considerable amount of heat [51, 52], thus leading to morerapid heating rates.

It has been suggested that, the more complex a material is, the moredifficult it is to prepare by using microwave-assisted synthesis. Inmore complex systems, very good diffusion is required to uniformlydisperse three or more cations throughout the sample during thesynthesis. The usual solution to this problem is to combine microwaveirradiation with other methods, such as sol-gel or combustion synthesis,as has been done for the synthesis of complex perovskites, such asLa_(0.8)Sr_(0.2)Fe_(0.5)Co_(0.5)O₃ or CaCu₃Ti₄O₁₂ [16, 53].

MW energy was reported to achieve the sintering of stabilized zirconiain a multimode microwave furnace at 2.45 GHz [73, 74] with sinteringtemperatures reduced by ca. 100° C. compared to conventional sinteringmethods, and that a finer grain size was obtained [75]. Gadolinium-dopedceria (GDC) powder was reported to be successfully synthesized usinghydrothermal-MW methods with a resulting increase in the ionicconductivity of GDC in comparison with what was achieved usingconventional ceramic processing methods [76].

The use of microwaves for the sintering of SOFC electrodes has beenreported [77-82]. For example, Jiao et al compared microwave sinteringand conventional thermal sintering of anode supports, showing that theanode-supported microwave-sintered cell exhibited a higher initialperformance and lower polarization than conventional thermally-sinteredcells [77, 80, 82].

There is increasing interest in the art in providing improved methodsfor processing ceramic materials that provide materials exhibitingdesired electronic properties and which reduce fabrication costs. Hereinthe use of microwave processing for generating certain electrodematerials as well as for fabricating the anode-electrolyte-cathode ofSOFCs is examined.

SUMMARY OF THE INVENTION

The invention provides electrode material, i.e., electrocatalyticmaterial, having the formula I:La_(w)M_(x)Fe_(y)Cr_(z)O_(3-δ)  (I)where:M is Ca or a mixture of Ca and Sr where the molar ratio of Ca to Srranges from 1:1 to 100:1;w is 0.2 to 0.4;x is 0.6 to 0.8;y is 0.6 to 0.8;z is 0.2 to 0.4;w=x is 1;and y=z is 1.In specific embodiments, M is Ca.In specific embodiments:w is 0.27 to 0.33;x is 0.67 to 0.73;y is 0.67 to 0.73; andz is 0.27 to 0.33.In specific embodiments:w is 0.29 to 0.31;x is 0.69 to 0.71;y is 0.69 to 0.1; andz is 0.29 to 0.31,where δ represents oxygen deficiency

In specific embodiments, w is 0.3; x is 0.7; y is 0.7; and z is 0.3.

In specific embodiments, the electrode material is a perovskite of theabove formula.

In specific embodiments, M is a mixture of Ca and Sr. More specificallyin an embodiment, the molar ratio of Ca to Sr is 1:1. In otherembodiments, the molar ratio of Ca to Sr is 2:1, 3:1, 4:1 or 5:1. Yetmore specifically, the molar ratio of Ca to Sr is 10:1.

In a preferred embodiment, in the electrode material, M is Ca. In apreferred embodiment, the electrode material isLa_(0.3)Ca_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ).

In a specific embodiment, the electrode material of the invention has anatomic % composition of;

La (15±0.5),

Ca (34.5±1),

Cr (15±0.5), and

Fe (35±1).

The invention further provides electrodes which comprise an electrodematerial of this invention. In a specific embodiment, such electrodesare formed as a layer on a solid oxide electrolyte. In a specificembodiment, the electrode is a fuel electrode, particularly for an SOFCor a reversible solid oxide fuel cell. In a specific embodiment, theelectrode is an air or oxygen electrode, particularly for an SOFC or anRSOFC.

The invention further provides electrodes which consist of an electrodematerial of this invention. In a specific embodiment, such electrodesare formed as a layer on a solid oxide electrolyte. In a specificembodiment, the electrode is a fuel electrode, particularly for a solidoxide fuel cell or a reversible solid oxide fuel cell. In a specificembodiment, the electrode is an air or oxygen electrode, particularlyfor an SOFC or a RSOFC.

The invention further provides electrodes which comprise an electrodematerial of this invention as the electrocatalytic material of theelectrode. Such electrodes may contain other supporting ornon-electrocatalytic active materials. In a specific embodiment, suchelectrodes are formed having at least one layer of electrocatalyticmaterial on a solid oxide electrolyte. In a specific embodiment, theelectrode is a fuel electrode, particularly for a solid oxide fuel cellor a reversible solid oxide fuel cell. In a specific embodiment, theelectrode is an air or oxygen electrode, particularly for an SOFC or aRSOFC.

In specific embodiments, crystals of the MIEC electrode material offormula I exhibit nanosized twinned domains as assessed in HTREM images.The appearance of these domains is associated with the pseudo-cubicnature of these materials. Furthermore, the presence of such domainsavoids the formation of tetrahedral chains and therefore the formationof undesired brownmillerite-type defects.

In specific embodiments, electrode layers range in thickness from 5 nmto 50 micron. In more specific embodiments, electrode layers range inthickness from 5 nm to 1 micron, from 50 nm to 50 micron, from 50 nm to1 micron, from 1 micron to 10 micron.

In specific embodiments, layers of electrocatalytic material range inthickness from 5 nm to 50 micron. In more specific embodiments, layersof electrocatalytic material range in thickness from 5 nm to 1 micron,from 50 nm to 50 micron, from 50 nm to 1 micron, or from 1 micron to 10micron.

The invention further provides solid oxide fuel cells having anelectrode which comprises an electrode material of the invention.

The invention further provides a reversible solid oxide fuel cell havingan electrode which comprises an electrode material of the invention.

The invention also provides methods for generating electricity whichcomprises operating a solid oxide fuel cell having at least oneelectrode comprising an electrode material of the invention.

The invention also provides methods for generating electricity and heator employing electricity to generate a fuel which comprises selectivelyoperating a reversible solid oxide fuel cell having an electrode of theinvention comprising an electrode material of the invention to generateelectricity or to generate a fuel.

The invention also provides methods wherein the solid oxide fuel cell orreversible solid oxide fuel cell is efficiently operated in the presenceof a fuel (e.g., CO₂, CO, H₂ and H₂O) containing hydrogen sulfide

In specific embodiments, electrode materials of the invention areprepared by microwave-assisted methods. In particular, electrodematerials of the invention are prepared by microwave-assistedcombustion, microwave-assisted co-precipitation or a microwave-assistedsol-gel method.

In additional embodiments, a microwave-assisted method for preparing thefull SOFC is provided. More specifically, the method includes a step ofproviding a layer of an anode material on one surface of a solidelectrolyte and a step of providing a layer of a cathode material on anopposite surface of a solid electrolyte and sintering the electrolytewith anode and cathode layers by application of microwave energy. In aspecific embodiment, the solid electrolyte has thickness of 10 nm to 2mm. In a more specific embodiment, the solid electrolyte has a thicknessof 0.5 to 2 mm. In a more specific embodiment, the solid electrolyte hasa thickness of 0.75 to 1.25 mm. In a more specific embodiment, the solidelectrolyte has a thickness of 0.9 to 1.1 mm.

In specific embodiments, the electrodes and fuel cells of the inventioncan exhibit tolerance to sulphur, e.g., up to 10 ppm H₂S, present infeed gases. In specific embodiments, the electrodes and fuel cells ofthe invention can exhibit decreased levels of carbon deposition (coking)compared to currently employed electrode materials.

In an embodiment, the method includes microwave-assisted processing ofanode and cathode materials prior to fabrication of the SOFC byapplication of microwave energy.

In a specific embodiment, the SOFC is a symmetric SOFC wherein the samematerial is employed for both the cathode and anode. In a specificembodiment, the anode and/or cathode material is a compound of formulaI. In a more specific embodiment, the anode and/or cathode material isLa_(0.3)M_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ). In a specific embodiment, theelectrolyte is gadolinium doped ceria (GDC) or yttria stabilizedzirconia. In a specific embodiment, the electrolyte has thickness of 10nms to 2 mm. In a more specific embodiment, the electrolyte hasthickness from 10 nm to 50 micron. In a more specific embodiment, theelectrolyte has a thickness of 0.5 to 2 mm. In a more specificembodiment, the electrolyte has a thickness of 0.75 to 1.25 mm. In amore specific embodiment, the electrolyte has a thickness of 0.9 to 1.1mm.

The invention further provides a method for preparation of SOFC andRSOFC cells which comprises sintering a solid oxide fuel cell (SOFC) ora reversible solid oxide fuel cell (RSOFC) by applying a slurry or pasteof a first and second electrode material to first and second oppositesurfaces of a solid electrolyte to form a first and second electrodelayer; and irradiating the resulting solid electrolyte with first andsecond layers with microwave radiation. More specifically, microwaveirradiation is conducted to reach a target temperature ranging from 600to 900° C. More specifically, microwave irradiation is conducted using aramp time ranging from 20 to 60 minutes to the target temperature. Morespecifically, microwave irradiation is maintaining at the targettemperature for 15 to 30 minutes. In a specific embodiment, theelectrode materials are those of formula I. In a specific embodiment,the solid electrolyte is GDC.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Rietveld refinement of LCFCr powder (synthesized by theconventional combustion method) X-ray diffraction patterns observed (reddotted lines), refined (black solid lines), and their difference (bottomline). Green vertical bars indicate the X-ray reflection positions.

FIGS. 2A and 2B. In situ high temperature XRD patterns from 25-1100° C.in air (FIG. 2A). Cell parameters a, b and c and unit cell volume as afunction of temperature (FIG. 2B).

FIG. 3. HRTEM image of LCFCr crystals in a powder sample (prepared bythe combustion method) along the [101] zone axis and the correspondingdigital diffraction pattern.

FIGS. 4A and B Impedance spectra of LCFCr at 800° C., 750° C., 700° C.,650° C. and 600° C. (FIG. 4A). The impedance response was obtained instagnant air at the OCP. Equivalent circuit used for data fitting (FIG.4B)

FIG. 5. Arrhenius plot of total polarization resistance (Rp) resistancevs. 1/T for LCFCr air electrode, screen-printed on GDC electrolyte andmeasured at the OCP in stagnant air over a temperature range of 600-800°C.

FIG. 6. Potentiostatic response of LCFCr tested at 800° C. and 0.4 V for100 h in stagnant air. Inset shows the OCP impedance spectra, in air,collected before and after the potentiostatic measurements.

FIG. 7. Potentiostatic response of LCFCr tested at 800° C. and −0.4 Vfor 100 h in stagnant air. Inset shows the OCP impedance spectracollected before and after the potentiostatic measurements in stagnantair.

FIGS. 8A-8C. Back-scattered electron (BSE) images of the cross-sectionof the LCFCr/GDC electrolyte interface after 100 hours of cell testingat 0.4 V anodic and cathodic overpotential at 800° C. (FIGS. 8A and 8B).FIG. 8C is a back-scattered electron (BSE) image of the cross-section ofthe LCFCr/GDC electrode/electrolyte interface in a cell beforeelectrochemical testing.

FIGS. 9A-C. Comparison of XRD patterns of LCFCr, synthesized by (FIG.9A) the regular combustion method (Method 1) and two microwave-relatedmethods, (FIG. 9B) by the microwave-combustion method (Method 2), and(FIG. 9C) by the microwave-assisted sol-gel method (Method 3) andcalcined at three different temperatures, 700° C., 900° C. and 1000° C.

FIGS. 10A and 10B. Rietveld refinement of powder X ray diffractionpatterns for LCFCr observed (red dotted lines), refined (black solidlines), and their difference (bottom line). Vertical bars indicate theX-ray reflection positions. The patterns are for LCFCr powder (FIG. 10A)synthesized by the microwave-combustion method (Method 2) and (FIG. 10B)by the microwave-assisted sol-gel method (Method 3).

FIGS. 11A and 11B. SEM images of LCFCr powders formed using (FIG. 11A)microwave-assisted combustion synthesis (Method 2) and (FIG. 11B)microwave-assisted sol-gel synthesis (Method 3).

FIGS. 12A and 12B. HRTEM images of LCFCr crystals along the [101] zoneaxis and the corresponding diffraction patterns for powders formed by(FIG. 12A) microwave-assisted combustion synthesis (Method 2) and (FIG.12B) microwave-assisted sol-gel synthesis (Method 3).

FIG. 13. OCP impedance data for symmetrical full cell based onLa_(0.3)M_(0.7)Fe_(0.7) Cr_(0.3)O_(3-δ) (M=Sr, Ca) electrodes, at 800°C., showing the Nyquist and Bode (inset) plots, all in wet 30% H₂/N₂ gasmixtures at the fuel electrode and with air exposure at the O₂electrode.

FIG. 14. OCP impedance data for symmetrical full cell based onLa_(0.3)Ca_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ) (LCFCr) electrodes at 800° C.and showing the Nyquist plot, all in wet 30% H₂/N₂ gas mixtures at thefuel electrode and with air or O₂ exposure at the O₂ electrode.

FIG. 15. Performance plot for symmetrical full cell based onLa_(0.3)M_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ) (M=Sr, Ca) electrodes andoperated at 800° C., all in wet 30% H₂/N₂ gas mixture at the fuelelectrode and with air exposure at the O₂ electrode.

FIG. 16. OCP impedance data for symmetrical full cell based on LCFCrelectrodes, at 800° C., showing the Nyquist plots in wet 30% H₂/N₂, 15%H₂+15% CO, or 30% CO gas mixtures at the fuel electrode and with airexposure at the O₂ electrode.

FIG. 17. Performance plot for symmetrical full cell based on LCFCrelectrodes, operated at 800° C. in wet 30% H₂/N₂, 15% H₂+15% CO, or 30%CO gas mixtures at the fuel electrode and with air exposure at the O₂electrode.

FIGS. 18A-E. OCP and polarized EIS response for symmetrical full cellbased on LCFCr electrodes at 800° C., with wet 30% H₂/N₂ with or without9 ppm H₂S fed to the fuel electrode and air fed to the O₂ electrode,showing the Nyquist plots acquired at (FIG. 18A) the OCP, (FIG. 18B)−100 mV vs the cell voltage at open circuit, (FIG. 18C) −300 mV vs. thecell voltage at open circuit, and (FIG. 18D) the correspondingresistances obtained from the fitted Nyquist plots using theRs(R_(HF)/CPE_(HF))(R_(LF)/CPE_(LF)) equivalent circuit model and the %Rp change (FIG. 18E).

FIGS. 19A and B. Effect of 9 ppm H2S exposure and removal on LCFCr anodeactivity as a function of polarization at 800° C., showing the currentversus time (i/t) plots at (FIG. 19A) −100 mV and (FIG. 19B) −300 mV vs.the full cell voltage at open circuit.

FIG. 20A is a cross-sectional SEM view of LCFCr electrode in Cell 1(blank) (with MW prepared powders followed by furnace sintering of thecell).

FIG. 20B is a cross-sectional SEM view of LCFCr electrode of FIG. 20Aafter electrochemical testing.

FIG. 21A is a cross-sectional SEM view of LCFCr electrode in Cell 3(with MW prepared powders followed by microwave sintering of the cell).

FIG. 21B is a cross-sectional SEM view of LCFCr electrode of FIG. 21Aafter electrochemical testing.

FIG. 22A shows impedance spectra of Cells 2 (circles) and 3 (squares)prepared from MW prepared powders followed by MW sintering of the cellsat 700 and 900° C., respectively. These are compared with that of a cellsintered using a conventional furnace at 1100° C. (Cell 1, triangles).LCFCr was deposited on either side of a 1 mm thick GDC electrolyte disc.The impedance response was obtained in stagnant air at the OCP and 800°C.

FIG. 22B is the equivalent circuit used for fitting data of FIGS. 22Aand 23.

FIG. 23 shows impedance spectra of Cells 4, 5 and 6 prepared from MWprepared powders followed by MW sintering of the cells at 850, 850 and600° C., respectively. LCFCr was deposited on either side of a 250 μmthick GDC electrolyte disc. The impedance response was obtained instagnant air at the OCP and 800° C. The equivalent circuit used for datafitting is that shown in FIG. 22B.

FIG. 24 illustrates an RSOFC of this invention. Exemplary reactions atthe oxygen and fuel electrode are shown.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to certain mixed metal oxide materials which areuseful as the active material in electrodes of solid oxide fuel cellsand particularly in reversible solid oxide fuel cells, either as anodesor cathodes, therein. In a specific embodiment, the electrode materialsherein can be used to make symmetrical solid oxide fuel cells where theelectrode material of the anode is the same as in the cathode. Theinvention further relates to the use of microwave-assisted processingand microwave sintering for preparation of SOFC electrodes and SOFC.

As is known in the art, solid oxide fuel cells convert energy in fuelsto electrical energy and heat. These fuel cells can be operated inreverse (as an electrolyzer) using electrical energy to convert amolecule, such as water, to a fuel, such as hydrogen and O₂. Reversiblecells operate in both modes. In a SOFC an oxidizing material, typicallyair or oxygen, is in contact with the cathode of the cell and the fuelis in contact with the anode of the cell. During fuel cell operation,oxygen ions are transported from the cathode to the anode to oxidize thefuel to form water or if carbon monoxide is present to form carbondioxide. SOFC cells typically operate at temperatures between about 650to 950° C. The electrodes are electrically connected and operationgenerates a current between the electrodes. In reverse mode, electricalenergy is used to produce oxidant and fuel. A reversible SOFC cell(RSOFC or Solid oxide electroyzer cell [SOEC]) can be operated in bothmodes. The present invention provides electrode materials that can beused as electrodes (anodes or cathodes or both) in SOFC cells, forexample as air or oxygen electrodes, or as electrodes in reversible SOFCcells.

FIG. 24 illustrates an RSOFC (10) of this invention. Exemplary reactionsat the oxygen and fuel electrode are shown. The solid electrolyte (2) ispositioned between two electrode layers (6 and 6′, the oxygen and fuelelectrode, respectively). Optional buffer layers may be provided (4 and4′). Current collector layers are provided for each electrode (8). In asymmetric cell, 6 and 6′ are made of the same electroactive material. Inthe present invention, the electroactive material is am MIEC material offormula I. The optional buffer layers may be of the same or differentmaterials. The optional buffer layer is made of a material that isdifferent from that of the solid electrolyte and the electrode material.In an embodiment, the buffer layers are present. In an embodiment, thebuffer materials 4 and 4′ are the same materials. Exemplary buffermaterials include among others lanthanide-doped ceria, such assamarium-doped ceria, lanthanum-doped ceria or gadolinium-doped ceria.

As is known in the art, a SOFC or RSOFC comprises an anode, a cathodeand a dense ionically conductive solid oxygen electrolyte between theanode and the cathode. Optionally, a buffer layer is positioned betweenthe anode and the electrolyte and/or between the cathode and theelectrolyte. In an embodiment, the anode and/or the cathode is providedas a layer on one side of a layer of electrolyte, with the otherelectrode, the cathode or the anode, being provided on the other side ofthe layer of electrolyte. Oxygen anions pass through the electrolytelayer from the cathode to anode or the reverse, dependent upon the modein which the cell is operated. The optional buffer can be provided as alayer between the layer of anode material and the electrolyte and/orbetween the layer of cathode material and the electrolyte. The electrodelayers optionally comprise a combination of the MIEC material of formulaI with a second MIEC material, wherein the second MIEC material is aminor component, less than 20% by weight of the total weight ofelectrode. The electrode layer optionally comprises a combination of theMIEC material of formula I with an electronically conductive material(not an MIEC).

In a specific embodiment, the RSOCF cell of FIG. 24 is formed bygenerating a first and a second electrode layer on opposite surfaces ofa dense solid electrolyte. The

The RSOCF of FIG. 24 is illustrated in a planar cell configuration. Itwill be apparent to those of ordinary skill in the art that other cellconfigurations maintaining the same relative relationship of electrolyteand electrodes can be employed for example, a tubular configuration canbe employed where the solid electrolyte is, for example, in the form ofa tube or a one-end closed tube.

In an embodiment, SOFC and RSOFC cells of the invention are operatedover a temperature range of 600 to 850° C. In other embodiments, thecells are operated over a temperature range of 650- to 800° C. In otherembodiments, the cells are operated over a temperature range of 750 to800° C.

U.S. Pat. No. 8,354,011 relates to reversible electrodes for solid oxideelectrolyzer cells (SOEC). This patent provides a description ofelectrodes for such cells and the operation of such cells. Such a planarconfiguration can be employed in SOFC and SOEC of this invention. Thispatent is incorporated herein by reference in its entirety fordescription of solid electrolytes, anodes and cathodes and methods ofmaking cells. Anodes described therein can be employed in devices ofthis invention in combination with electrodes described herein.

Chen, M. et al. (2013) J. Power Sources 236:68-79 describes the use ofcertain Sr-rich chromium ferrites for symmetrical solid oxide fuelcells. In particular, the use of La_(0.3)Sr_(0.7)Fe_(0.7)C_(0.3)O_(3-δ)is described. The reference is incorporated by reference herein in itsentirety for description of the preparation and properties of theelectrode materials therein and for descriptions of construction of fuelcells and applications of fuel cells.

U.S. Pat. No. 8,617,763 provides a description of certain SOFC cells andin particular a certain type of anode useful in such cells. Anode,cathode and electrolyte materials described therein can be employed inthe devices of the present invention. The reference is incorporated byreference herein in its entirety for description of the preparation andproperties of the electrode materials therein and for descriptions ofconstruction of fuel cells and applications of fuel cells.

Electrode materials of the invention are those of formula:La_(w)M_(x)Fe_(y)Cr_(z)O_(3-δ)where:M is Ca or a mixture of Ca and Sr where the molar ratio of Ca to Srranges from 1:1 to 100:1;w is 0.2 to 0.4;x is 0.6 to 0.8;y is 0.6 to 0.8;z is 0.2 to 0.4;w=x is 1;and y=z is 1.

A preferred electrode material isLa_(0.3)Ca_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ). In a particularly preferredembodiment the electrode material is a single phase material having nodectable second or other additional phase. In an embodiment, theelectrode material is substantial single phase material, with less than10% by weight of a second or other additional phase or preferably havingless than about 5% by weight of a second or other additional phase andmore preferably having less than about 2% by weight of a second or otheradditional phase In specific embodiments, the electrode material is aperovskite.

Various methods can be employed to prepare the mixed metal oxidecompounds of the invention. For example, the following methods can beused:

A. Microwave Method Combined with a Sol-Gel Methodology.

Microwave method and a sol-gel methodology can be combined to makeelectrode materials of the invention, for exampleLa_(0.3)Ca_(0.7)Fe_(0.7)C_(0.3)O_(3-δ) (LCFC). Equimolar amounts ofmetal nitrates are dissolved in distilled water and a saturatedpolyvinyl alcohol (PVA) solution is added as the complexing agent. Theamount of PVA added is such that the ratio of the total number of molesof cations to that of PVA is 1:2. Then the final solution is maintainedat 80° C. for 1.5 h to form a viscous gel solution. This gel is thenirradiated with microwaves (up to 30 min) in a porcelain crucible placedinside another larger one filled with mullite. The microwave sourceoperates at 2.45 GHz frequency and 800 W power and is uniquely able tohandle the conditions needed. The polymeric and sponge-like-precursor isthen calcined in air at 1000° C. for 6 h in order to decompose theorganic remnants, rendering a black powder as the final product.

B. Microwave-Assisted Combustion

Metal nitrates are mixed in stoichiometric proportions, and then waterand glycine are added. The sample is introduced into the microwavefurnace at 2.45 GHz frequency and 800 W power for 30 minutes. When thewater is evaporated, combustion occurred and a flame is observed insidethe microwave furnace for 10 minutes. Then the sample is calcined in airat 900° C. for 6 h in order to decompose the organic remnants, renderinga black powder as the final product.

C. Microwave-Assisted Co-Precipitation

Metal nitrates are mixed in stoichiometric proportion, 25 ml of aceticacid are added, and then the mixture is stirred and heated at 60° C. for2 hours. When the nitrate vapors are evaporated, a gel formed and thenit is introduced into the microwave furnace at 2.45 GHz frequency and800 W power for 30 minutes, followed by calcination at 900° C.

D. Regular Combustion Method

When synthesized using the regular combustion method (Method 1), themetal nitrates are mixed in stoichiometric proportions and dissolved indeionized water. A 2:1 mole ratio of glycine to the total cation contentis used. Solutions are slowly stirred on a hot plate until auto-ignitionand self-sustaining combustion occurred. The sample is first ground andthen calcined in air at 1200° C. for 12 hours.

The electrode materials of the invention can be employed in any SOFC orRSOFC configurations and are particularly useful in those configurationswhich employ electrode layers.

Solid electrolytes useful in the invention include stabilized zirconia,including yttrium stabilized zirconia and scandia stabilized zirconia,doped ceria, including gadolidium-doped ceria or samarium-doped ceria,and certain mixed metal oxides such as LSGM (lanthanum strontium galliummagnesium oxide). One of ordinary skill in the art knows how to selectsolid oxide electrode s appropriate for use in SOFC and RSOFC devices.In a specific embodiment, SOFC and RSOFC of the invention employ a LSGMsolid electrolyte with optional buffer layers of lanthanum-doped ceria(LDC) or GDC. In an embodiment, the LSGM isLa_(0.8)Sr_(0.2)Ga_(0.8)Mg_(0.2)O_(3-δ). In a specific embodiment, SOFCand RSOFC of the invention employ a LSGM solid electrolyte with optionalbuffer layers of lanthanum-doped ceria or gadolidium-doped ceria.

In specific embodiments herein, the SOFC and RSOFC cells are symmetricwherein the anode and cathode materials are the same and are electrodematerials of this invention. In alternative embodiments, alternativeanode having alternative electrode materials can be used in combinationwith cathodes having electrode materials of this invention. Alternativeanode materials include, among others, perovskite mixed metal oxidematerials other than those of this invention, e.g.,La_(0.3)Sr_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ), cermets having a metal phase,such as a nickel or nickel oxide phase, and a ceramic phase, such asdoped ceria (samaria or gadolinium-doped), and/or stabilized zirconia.

In alternative embodiments, alternative cathodes having alternativeelectrode materials can be used in combination with anodes havingelectrode materials of this invention. Alternative cathode materialsinclude among others, perovskite mixed metal oxide materials other thanthose of this invention, e.g., La_(0.3)Sr_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ),electron conducting phases (e.g., nickel oxide and magnesium oxide).

One of ordinary skill in the art in view of what is known in the artabout electrode materials useful in SOFC or RSOFC application can selectamong known electrode materials for alternative electrode materials thatare useful in combination with the electrode materials of thisinvention.

Anodes and cathodes may be formed a one or more layers on a surface of asolid electrolyte.

Solid electrolyte can be in a planar layer configuration with one sideof the electrolyte layer containing a layer of anode material and theother a layer of cathode material. In symmetric cells the electrodelayers are the same materials.

SOFC and RSOFC electrodes are prepared by formation of at least onelayer of the electrode material on an appropriate substrate. In aspecific embodiment, an electrode is formed by application of a layer ofelectrode material on a surface of a solid oxide electrolyte material.In a preferred method of preparation of electrodes microwave sinteringis employed. The electrode material is screen printed onto the solidoxide electrolyte and it is irradiated at 900° C. for 20 minutes in aMilestone MultiFAST-6 sintering microwave. It was found that the bestperformance was for the sample irradiated at 900° C. and the cellperformance is comparable to the electrodes sintered using conventionalfurnaces at 1200° C. In an embodiment, microwave power employed forpreparation of cells ranges from 1000-1500 W. In an embodiment, MWfrequencies ranging from 0.3 to 30 GHZ are employed. In an embodiment,MW frequencies in the range of 2 to 3 GHX are used.

The SOFC and RSOFC of the invention can be formed into stacks, as isknown in the art. Stacks of such cells are provided by this invention.U.S. Pat. No. 8,663,869 provides examples of such fuel cell stacks. Thisreference is incorporated by reference herein in its entirety fordescription of the preparation of electrodes, construction of fuel cellsand fuel cell stacks and applications of fuel cells.

References 43, 54, 84 and 85 provide details of the examples providedherein. These references are specifically incorporated by referenceherein for disclosure of experimental details, preparation of electrodematerials, construction of RSOFC and SOFC, properties of materials andapplications of SOFC and RSOFC.

The invention also provides a microwave-assisted method for preparationof SOFC and RSOFC cells. The method involves sintering of the fuel cellsby irradiation with microwave radiation. In embodiments, the frequencyof the microwave radiation employed ranges from 0.3 GHz to 30 GHz. In aspecific embodiment, the frequency of the microwave radiation employedranges from 1 to 5 GHZ and more preferably from 2 to 3 GHz. In anembodiment, microwave irradiation was apply to the cell using ramp timeranging from 20 to 60 minutes to target temperatures which range from600 to 900° C. Once target temperature is reached the microwaveirradiation is continued at the target temperature for a selected time.In specific embodiments, the irradiation time at target temperatureranges from 15 to 30 minutes.

Cell construction proceeds by initial coating of a paste or slurry ofselected electrode material on a first and opposite second surface of asolid electrolyte. The solid electrolyte is in the form of a densesubstrate which can be planar or tubular, for example. To form asymmetrical cell, the same electrode material is provided in a layer onthe two opposing surfaces of the electrolyte. In a specific embodiment,the first and second electrode layers are screen printed on the solidelectrolyte. In an embodiment, the solid electrolyte with electrodelayers is subjected to microwave radiation to sinter the entire cell. Ina specific embodiment, the cell is prepared using one microwaveirradiation step. In another embodiment, the electrode material isapplied to both opposing surfaces of a solid electrolyte, where the twoopposing surfaces have already been provided with layers of a buffermaterial, and the cell is then subjected to one step of microwaveirradiation. The cell preparation is optionally conducted with multiplesteps of microwave irradiation. For example, in a first step, bufferlayers are applied to one or both opposing surfaces of the solidelectrolyte and a first step of microwave irradiation is applied asdescribed above. Thereafter, the electode layers are applied to theelectrolyte surfaces already containing the one or more buffer layers,where application of electrodes is preferably by screen printing. Themultilayer cell is then subjected to a second microwave irradiation tocomplete sintering of the cell.

In specific embodiments, the electrode material is a material of formulaI. In specific embodiments, the target temperature for microwaveirradiation is greater than 800° C. In more specific embodiments, thetarget temperature for microwave irradiation is 900° C. In embodiments,the temperature is ramped to the target temperature over 20 to 40minutes. In specific embodiments, the temperature is ramped to thetarget temperature over 25 to 35 minutes. In a specific embodiments, thetemperature is ramped to the target temperature over 30 minutes. In anembodiment, once at target temperature, microwave irradiation iscontinued at the target temperature for 15 to 25 minutes. In a morespecific embodiment, microwave irradiation at the target temperature iscontinued for 20 minutes. Conductive layers are applied to the outersurfaces of the electrodes to provide current collectors. In a specificembodiment, current collectors are made of the same material. In aspecific embodiment, the current collector is a layer of Au.

In specific embodiments, the powders used to prepare the electrodes arethemselves prepared by microwave-assisted methods. In specificembodiments, the powders used to prepare the solid electrolyte arethemselves prepared by microwave-assisted methods. In a specificembodiment, the solid electrolyte is prepared using microwave sintering.

It is noted that the particle size of materials in fuel cells preparedby microwave sintering have a particle size that is smaller than cellsprepared by conventional oven sintering. More specifically, in anembodiment, the average particle size of the materials in the fuelcells, and particularly electrode materials, is less than 75 nm. In anembodiment, the average particle size of the matetrials in the fuelcells, and particularly electrode materials, is in the range from 25 to75 nm and more preferably in the range from 45 to 55 nm.

When a Markush group or other grouping is used herein, all individualmembers of the group and all combinations and subcombinations possibleof the group are intended to be individually included in the disclosure.Every formulation or combination of components described or exemplifiedcan be used to practice the invention, unless otherwise stated. Specificnames of compounds are intended to be exemplary, as it is known that oneof ordinary skill in the art can name the same compounds differently.

One of ordinary skill in the art will appreciate that methods, deviceelements, starting materials, and synthetic methods other than thosespecifically exemplified can be employed in the practice of theinvention without resort to undue experimentation. All art-knownfunctional equivalents, of any such methods, device elements, startingmaterials, and synthetic methods are intended to be included in thisinvention. Whenever a range is given in the specification, for example,a temperature range, a time range, or a composition range, allintermediate ranges and subranges, as well as all individual valuesincluded in the ranges given are intended to be included in thedisclosure.

The invention illustratively described herein suitably may be practicedin the absence of any element or elements, limitation or limitationswhich is not specifically disclosed herein.

Without wishing to be bound by any particular theory, there can bediscussion herein of beliefs or understandings of underlying principlesrelating to the invention. It is recognized that regardless of theultimate correctness of any mechanistic explanation or hypothesis, anembodiment of the invention can nonetheless be operative and useful.

All references throughout this application, for example patent documentsincluding issued or granted patents or equivalents; patent applicationpublications; and non-patent literature documents or other sourcematerial; are hereby incorporated by reference herein in theirentireties, as though individually incorporated by reference.

All patents and publications mentioned in the specification areindicative of the levels of skill of those skilled in the art to whichthe invention pertains. References cited herein are incorporated byreference herein in their entirety to indicate the state of the art, insome cases as of their filing date, and it is intended that thisinformation can be employed herein, if needed, to exclude (for example,to disclaim) specific embodiments that are in the prior art. Forexample, when a compound is claimed, it should be understood thatcompounds known in the prior art, including certain compounds disclosedin the references disclosed herein (particularly in referenced patentdocuments), are not intended to be included in the claim.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention in the useof such terms and expressions of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed. Thus, it should be understood that although thepresent invention has been specifically disclosed by preferredembodiments and optional features, modification and variation of theconcepts herein disclosed may be resorted to by those skilled in theart, and that such modifications and variations are considered to bewithin the scope of this invention.

THE EXAMPLES Example 1

Synthesis and Characterization

La_(0.3)Ca_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ) (LCFCr) powders were synthesizedby the combustion method. Metal nitrate precursors were mixed instoichiometric proportions and dissolved in deionized water. Glycine (a2:1 mole ratio of glycine to the total cation content) was added.Mixtures were slowly stirred on a hot plate until auto-ignition andself-sustaining combustion occurred. The sample was then ground andcalcined in air at 1200° C. for 12 hours.

Materials were purchased from Alfa Aesar as follows:

Glycine (99.5%); La (NO₃)₃.6H₂O (99.9%); Sr(NO₃)₂ (99.0%); Ca(NO₃)₂(99.0%); Cr(NO₃).9H₂O (98.5%); and Fe(NO₃)₃. 9H2O (98-101%).

X-ray diffraction (XRD) patterns of all samples synthesized in thisexample were collected using a Philips X'Pert PRO ALPHA1 of PanalyticalB.V. diffractometer with Cu K_(α1) monochromatic radiation (λ=1.54056Å). The diffractometer was equipped with a primary curved Ge111 primarybeam monochromator and a speed X'Celerator fast detector, operating at45 kV and 40 mA. XRD patterns were collected in the 2θ range of 5-120°at room temperature with a step size of 0.017° and 8 s counting time inorder to ensure sufficient resolution for structural refinement.

Powder X-ray Thermodiffraction patterns were collected on an X'Pert PROMPD diffractometer with a high temperature reactor chamber Anton PaarHTK1200 camera, using Cu Kα radiation. The measurements were carried outat between room temperature and 1100° C. The standard working conditionswere a 2θ range of 10-70° with an angle step size of 0.033° and a 25 scounting time. Sample was heated to the target temperatures at a ramprate of 5° C./min and stabilized in air for 40 min prior to themeasurements. After that, the sample was cooled to RT and XRD patternswere acquired again in order to determine the phase stability of theLCFCr material under heating and cooling conditions.

Fullprof Software was employed to carry out structural refinements fromconventional XRD patterns using the Rietveld method. This method ofrefining the powder diffraction data was used to determine the crystalstructure. Zero shift, lattice parameters, background, peak width, shapeand asymmetry, atomic positions and isotropic temperature factors wereall refined. The Thompson-Cox-Hastings pseudo-Voigt convoluted withaxial divergence asymmetry function was used to describe the peak shape.Linear interpolation between set background points with refineableheights was used afterwards. The values were refined to improve theagreement factors.

All samples investigated by scanning electron microscopy (SEM) werefirst sputter-coated with Au in an EMITECH K550 apparatus.Field-emission SEM (FE-SEM) was performed using a JEM 6335 F electronmicroscope with a field-emission gun operating at 10 kV. The FE-SEM wasalso equipped with a LINK ISIS 300 detector for the energy-dispersiveanalysis of the X-rays (XEDS). SEM imaging of the cells and attachedelectrode layers was carried out using a Zeiss Σigma VP field emissionSEM.

High resolution transmission electron microscopy (HRTEM) analysis of theLCFCr powders was performed using a JEOL 3000F TEM, operating at 300 Kv,yielding information limit of 1.1 Å. Images were recorded with anobjective aperture of 70 μm centered on a sample spot within thediffraction pattern area. Fast Fourier Transforms (FFTs) of the HRTEMimages were carried out to reveal the periodic image contents using theDigital Micrograph package.

Cell Fabrication and Testing

The LCFCr powders obtained from the regular combustion method weremilled (high energy planetary ball mill, Pulverisette 5, Fritsch,Germany) in an isopropanol medium at a rotation speed of 300 rpm for 2 husing zirconia balls. The electrolyte-supported symmetrical cell wasconstructed with a GDC electrolyte (1 mm thick) as the substrate. Theelectrolyte was fabricated by pressing the GDC powder under 200 MPapressure and conventional oven sintering at 1400° C. for 4 h. The ca. 30μm thick LCFCr electrodes were then screen-printed symmetrically (overan area of 0.5 cm²) onto both sides of the GDC support and fired at1000° C. for 2 h. Au paste (C 5729, Heraeus Inc., Germany) was paintedon both of the electrode layers to serve as the current collectors.

Electrochemical measurements to evaluate the cell performance wereperformed using the 3 electrode technique in air Impedance spectra werecollected under open circuit conditions, between 600° C. and 800° C.,using an amplitude of 50 mV in the frequency range of 0.01 to 65 kHzusing a Solatron 1287/1255 potentiostat/galvanostat/impedance analyzer.Other experiments involved the application of a 0.4 V anodic and −0.4 Vcathodic overpotential to the LCFCr working electrode vs. the referenceelectrode and measuring the current passed through the cell with time.Zview software was used to fit and analyze the impedance data.

X-Ray Diffraction and Rietveld Refinement of LCFCr Powders

XRD analysis of the La_(0.3)Ca_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ) (LCFCr)powders was performed and structural parameters for LCFCr were obtainedfrom the Rietveld-refined XRD data. The Rietveld refinement indicatedthat the synthesized LCFCr powders are a pure crystalline phase with anorthorhombic perovskite structure. FIG. 1 shows the Rietveld refinementfits for LCFCr, and a distorted perovskite structure with anorthorhombic symmetry (S.G. Pnma, #62) was confirmed. The unit cellvectors can be represented by √2a_(p)×2a_(p)×√2a_(p), where a_(p) refersto the simple cubic perovskite cell. The cell parameters were found tobe: a=5.4540(2) Å, b=7.7158(3) Å and c=5.4544(1) Å, while the refinementfit parameters for LCFCr were χ2=0.96, R_(p)=3.26, R_(wp)=4.29,R_(exp)=4.37 and R_(Bragg)=4. Although the orthorhombic unit cell seemsto be pseudo tetragonal, refinements were also performed in the P4/mmmspace group, but these yielded higher R values (χ2=4.03, R_(p)=5.53,R_(wp)=8.76, R_(exp)=4.37 and R_(Bragg)=5.59), while the latticeparameters when using this tetragonal group were: a=b=5.45441(1) Å andc=7.7092(1) Å. Thus, the P4/mmm space group was not used for the LCFCrelectrode material.

In order to determine the phase stability of the LCFCr material underheating and cooling conditions, in situ high temperature XRDmeasurements were performed from room temperature to 1100° C., and thenback to room temperature again, all in air. FIG. 2A shows that theorthorhombic structure is maintained over the full temperature range upto 1100° C., since peak splitting is not observed. Moreover, a shift ofall the characteristics peaks towards lower angles is observed, whichindicates an increase in the cell parameters with temperature. FIG. 2Bshows the cell parameters a, b and c, as well as the unit cell volume vstemperature, calculated from XRD data (FIG. 2A).

Table 1 shows the average thermal expansion coefficient calculated fromthe thermal XRD data (FIGS. 2A and 2B), using previously describedmethods [26]. The average TEC is 11.5×10⁻⁶ K⁻¹ for lattice parameter a,and 12.0×10⁻⁶ K⁻¹¹ for lattice parameters b and c. These values arecomparable to those reported for the well-known cathode material LSM(12.2×10⁻⁶ K⁻¹) [21-23] and noticeably lower than the TEC values forLSCF (16.3×10⁻⁶ K⁻¹) [21, 24]. The measured TEC values are alsoconsiderably lower than those for the Sr-rich perovskite,La_(0.3)Sr_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ) (LSFCr), previously developed[14]. More importantly, the measured thermal expansion coefficient (TEC)of LCFCr (Table 1) matches very well with the TEC of ceria (11.9×10⁻⁶K⁻¹¹) [11, 21, 25-31], which is an important requirement for minimizingdelamination of the electrodes from the electrolyte, and to avoidmechanical failure of the cell.

TABLE 1 Average thermal expansion coefficient (TEC) for LCFCr material,determined by in situ XRD analysis Thermal expansion Average TECparameters (×10⁻⁶ K⁻¹) Lattice parameter (a) 11.5 (25-1100° C.) Latticeparameter (b) 12.0 (25-1100° C.) Lattice parameter (c) 12.0 (25-1100°C.)TEM Analysis of LCFCr Powder

Transmission Electron Microscopy (TEM) analysis was also performed onthe LCFCr powder material. The cation composition, evaluatedsemi-quantitatively by X-ray energy dispersive spectroscopy in more thanten single crystals, is in good agreement with the theoreticalproportions of the elements in LCFCr, indicating the high purity of thepowder. High resolution TEM micrographs recorded along the same zoneaxis [101] show (FIG. 3) nano-sized twinned domains rotated by 90°. Theappearance of these domains can be associated with the pseudo-cubicnature of these materials. The presence of these domains can help toavoid the formation of tetrahedral chains and therefore the formation ofbrownmillerite-type defects [32]. Typically, raising the temperatureleads to a phase transition of brownmillerite to perovskite at hightemperatures, accompanied by a conductivity jump [33]. As mentionedearlier, perovskites exhibit a higher ionic conductivity thanbrownmillerites and hence they are better candidates for air electrodesin RSOFCs.

Electrochemical Performance of LCFCr as a Reversible Air Electrode OpenCircuit Studies

The electrochemical performance of the LCFCr material, synthesized bythe regular combustion method, was studied, with the impedance spectraof the LCFCr/GDC/LCFCr symmetrical half cells in air at 800, 750, 700,650 and 600° C. shown in FIG. 4A, all at the open circuit potential(OCP). From FIG. 4A two separable arcs are visible over the fullfrequency range. The best-fit equivalent circuit is shown in FIG. 4B,where Rs is the series ohmic resistance, the sum of R2 (high frequency)and R3 (low frequency) is the total polarization resistance (Rp), andthe CPEs are constant phase elements. Rs corresponds to the interceptsof the impedance arc with the real axis at high frequencies and arisesfrom the resistance to ion migration within the electrolyte, resistanceto electron transport within the cell components, and contactresistances [34]. Rp is the difference between the two real axisintercepts of the impedance arcs and CPE is a component that models thebehaviour of a an imperfect capacitor [35], with the associated nparameter being 1 for a perfect capacitor, 0 for a pure resistor, and0.5 for a Warburg element [36]. The high-frequency arc (R2) correspondsto the charge transfer process and the low-frequency arc (R3) has beenattributed previously in the literature [12, 37, 38] to oxygenadsorption and desorption on the electrode surface, combined with thediffusion of the oxygen ions.

As shown in Table 2, the R_(p) values are very small, 0.07, 0.33, 0.73,1.67 and 4.24 Ωcm² at 800, 750, 700, 650 and 600° C., respectively forLCFCr material. These values are lower than those reported for thewell-known cathode material LSCF (0.18 Ωcm² at 800° C.) [11, 39].However, these Rp values are comparable to what was reported forLa_(0.3)Sr_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ) (LSFCr), studied using a LSGMelectrolyte, giving an Rp value of 0.11 Ωcm² at 800° C. [14]. In termsof the capacitance values obtained from the cell examined in FIG. 4A,the high-frequency arc (R2) has a CPE-T value of ca. 10⁻¹ (Fs)^(1-n)/cm² and an associated CPE-P value of 0.72, while thelow-frequency arc (R3) also has a CPE-T value of ca. 10⁻¹ (Fs)^(1-n)/cm²), but an associated CPE-P value of 0.86, very close to thatof an ideal capacitor.

The Arrhenius plot of the total OCP polarization resistance for theLCFCr material in air, obtained from the data of FIG. 4A, is presentedin FIG. 5. According to the fitting parameters shown in Table 2, theresistance of the low frequency arc is approximately 90% of the total Rpand thus the activation energy associated with this

TABLE 2 Fitting parameters of the impedance data obtained in FIG. 4ATem- CPE- CPE- per- R_(LF) P R_(HF) P R_(p) Chi- ature (Ω · cm²) (LF) (Ω· cm²) (HF) (Ω · cm²) squared 800° C. 0.05 0.86 0.02 0.72 0.07 1.6 ×10−5 750° C. 0.21 0.52 0.12 0.71 0.33 1.1 × 10−4 700° C. 0.51 0.32 0.220.70 0.73 3.1 × 10−4 650° C. 1.18 0.27 0.49 0.67 1.67 3.5 × 10−4 600° C.2.99 0.22 1.24 0.61 4.24   4 × 10−4arc will be dominant. As shown in FIG. 5, good linearity of the plot ofthe polarization resistance versus the inverse of temperature isobtained. The derived activation energy (E_(a)) for the ORR is 125kJ/mol, which is lower than previously reported for well-known cathodematerials, such as LSM (173.7 kJ/mol [40, 41]) and LSCF (178.5 kJ/mol[42]) at the OCP in air. The lower Ea indicates that the LCFCr materialis a better catalyst for the ORR than LSM and LSCF. Furthermore,according to the literature, this range of activation energies suggeststhat oxygen diffusion in the gas phase is one of the slow steps of thereaction [36].Performance of LCFCr Under Anodic and Cathodic Polarization

To further investigate the medium-term electrochemical stability of theLCFCr air electrode for RSOFC applications, potentiostatic experimentsat 800° C., at overpotentials of 0.4 V (OER) and −0.4 V (ORR), wereperformed for 100 h. In FIG. 6, a degradation rate of 0.59 mA h⁻¹ isseen over 100 h at the anodic 0.4 V overpotential. Impedancemeasurements, however, show that R_(p) is very similar before (0.35 Ωcm²Ωcm²) and after (0.30 Ωcm²) the 100 h test at 0.4 V, demonstrating verygood medium-term stability of the LCFCr air electrode performance undertypical OER operating conditions.

These experiments were performed 24 days after commencing cell testing(FIG. 4) and some degradation of the cell performance has clearlyoccurred, as seen by comparing the results in FIGS. 6 and 7 with thosein FIG. 4A. However, the ohmic resistance (R_(s)) is the main cause ofthis degradation, having changed from 0.77 to 0.99 Ωcm², likely due tothe sintering of the current collectors. The shift of the summitfrequency (41 Hz before testing and 2.58 Hz after testing) may beconsistent with the densification of the Au current collector. Thus, themajority of the degradation seen in FIG. 6 is thus due to this increasein Rs. In support of this conclusion, our previously published WDXelemental map studies did not reveal an incompatibility issue betweenLCFCr and GDC [43].

The medium-term electrochemical stability of the LCFCr air electrodetowards the oxygen reduction reaction (ORR) was then investigated at a−0.4 V overpotential, again at 800° C. for 100 h. In FIG. 7, a loss incurrent of 0.67 mA h⁻¹ is seen over this time period, which indicatesthat LCFCr experiences a slightly faster degradation as an ORR catalystthan during the OER Impedance measurements performed before and afterthe potentiostatic experiment (FIG. 7) show that R_(p) increases from0.25 Ωcm² before cathodic polarization to 0.30 Ωcm² after the 100 h testat −0.4 V. However, Rs does not change, and, in fact, has the same valueas that before the anodic (+0.4 V) polarization experiment in FIG. 6.These observations show that LCFCr performs more poorly as an ORRcatalyst than during the OER. Furthermore, the fact that Rs in FIG. 7has recovered to its original OCP value before anodic polarization (FIG.6) demonstrates that LCFCr is an excellent air electrode for the OER,and that the loss in performance in FIG. 6 is not permanent (the lossesobserved here in Rs appear to be reversible). Sintering of the currentcollectors remains the most likely reason for the increase of Rs withtime. Furthermore, it is evident that, when the polarization wasswitched from +0.4 V (FIG. 6) to −0.4 V (FIG. 7), Rs fully recovered.Thus, it is plausible that dewetting of the gold current collector,which may have occurred as a result of sintering at +0.4 V, may havereversed upon the change of polarization direction. This is consistentwith the known effect of electrical potential on interfacial tensions[44, 45].

Overall, the LCFCr material is seen to be an excellent air electrode,giving Rp values in the range or even lower than the best SOFC cathodematerials discussed in the literature in this temperature range. Forexample, LSCF has exhibited an Rp value of 0.18 Ωcm² at the OCP [11] andLSFCu an Rp of 0.07 Ωcm², both at 800° C. [12].

Cell Microstructure

The typical microstructure of the cell, examined by back-scattered SEM,is shown after electrochemical testing in FIGS. 8A and 8B. The cellconsists of a 1 mm dense GDC electrolyte layer, with only one of the twoLCFCr electrode layers (˜30 μm thick) shown. A gold current collectorlayer is shown in the image (white phase in FIG. 8A), The LCFCr layerdisplays very good porosity at higher magnification (FIG. 8B). Forcomparison, FIG. 8C shows a somewhat higher magnification image(compared to that in FIG. 8A) of the microstructure and the interface ofa cell before electrochemical analysis.

The LCFCr/GDC interface after cell testing at both 0.4 V and −0.4 V,each for 100 h, is seen in FIGS. 8A and 8B to have retained a continuousgood contact between the LCFCr electrodes and the GDC electrolyte, withno delamination or cracking detected. Delamination of the oxygenelectrode from the electrolyte is the most common degradation and cellfailure issue for high temperature electrolysis cells [2]. TheLCFCr-based symmetrical cell did not show any electrode delamination(FIG. 8B) after long times under both anodic and cathodic polarization.

In this example, Ca was substituted for Sr in the A site ofLa_(0.3)Sr_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ) (LSFCr), to produce the mixedconducting perovskite material La_(0.3)Ca_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ)(LCFCr). LCFCr was prepared using the combustion method. XRD analysisshowed that the LCFCr powders are a pure crystalline phase, alsoconfirmed by TEM analysis, with an orthorhombic perovskite structure. Itwas also shown that the average TEC values match closely with that ofgadolinium-doped ceria (GDC), the electrolyte used here.

Electrochemical measurements showed very good performance with opencircuit potential (OCP) polarization resistances (Rp) comparable to whathas been reported for other well-known perovskites, used in air, at600-800° C. Further, the activation energy of the oxygen reaction atLCFCr, at the OCP, was found to be lower than literature values forother well-known air electrodes. Investigation of the medium-termelectrochemical stability of the LCFCr air electrode towards the OER(0.4 V) and ORR (−0.4 V) at 800° C. for 100 h showed that Rp hardlychanges during the OER, but increases by ca. 20% during the ORR. SEMimaging of the LCFCr/GDC interface showed no delamination or other formsof physical degradation of the cell after 100 hours at both 0.4 V and−0.4 V.

Example 2: Microwave-Assisted Synthesis of LCFCr

In this example, alternative powder processing methods are examined,with a primary focus on microwave-based synthesis that could both lowermaterial manufacturing costs and further enhance cathode performance forsolid oxide fuel cell applications.La_(0.3)Ca_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ) (LCFCr), formed usingconventional solid-state methods (Example 1), has been shown a usefulcatalyst for the oxygen reduction reaction. To further increase itsperformance in such applications, microwave methods were used toincrease the surface area of LCFCr and to decrease the processing time.It was found that the material could be obtained in crystalline form inonly 8 hours, with the synthesis temperature lowered by roughly 300° C.as compared to conventional oven methods.

Mixed ionic and electronic conducting oxides (MIEC), are of interest asmore durable as cathodes than conventional La_(1-x)Sr_(x)MnO₃ (LSM)materials. MIECs are usually synthesized by solid-state reactions, wherethe process involves multiple heating (≥1200° C.) and regrinding stepsto help overcome the solid-state diffusion barrier [16, 46]. The sol-gelmethod [47], the EDTA citrate complexing process [12], the auto-ignitionprocess [7], the Pechini method [9], and most commonly, combustionmethods [14] have been used to prepare MIECs.

In this example, the synthesis and characterization of LCFCr, formedusing three different methods, regular combustion (Method 1),microwave-assisted combustion (Method 2), and microwave-assisted sol-gelsynthesis (Method 3) is compared. A single phase material can besuccessfully synthesized using microwave-assisted methods and thecalcination temperature can be lowered by 200-300° C. using thisapproach.

Material Synthesis

La_(0.3)Ca_(0.7)Cr_(0.3)Fe_(0.7)O_(3-δ) (LCFCr) powders were synthesizedusing three different methods, the regular combustion method (Method 1,Example 1), microwave-assisted combustion (Method 2), andmicrowave-assisted sol-gel synthesis (Method 3). The combustion methoddescribed in Example 1 was used as Method 1.

LCFCr powders were also synthesized by microwave-assisted combustion(Method 2). Here, the metal nitrates and glycine were dissolved indeionized water using the metal cation proportions required to generatethe correct oxide stoichiometry. A 2:1 mole ratio of glycine to thetotal metal content was used. The stirred solutions were introduced intothe microwave furnace and exposed to a 2.45 GHz frequency and 800 Wpower for 30 minutes. When the water had evaporated, combustionoccurred. The sample was then calcined in air at 700° C., 900° C. and1000° C. for 8 h in order to decompose the organic remnants, rendering ablack powder as the final product.

In Method 3, microwave energy and a sol-gel methodology were combined toproduce the LCFCr powders, with the metal cation proportions used basedon the desired stoichiometry. Metal nitrates were dissolved in distilledwater and a saturated polyvinyl alcohol (PVA) solution was added toserve as the complexing agent. The amount of PVA added was such that theratio of the total number of moles of cations to that of PVA was 1:2.Then the final solution was maintained at 80° C. for 1.5 h to form aviscous gel. The gel was then microwave irradiated (up to 30 min) in aporcelain crucible placed inside another larger crucible filled withmullite. The microwave source operated at a 2.45 GHz frequency and 800 Wpower. The polymeric and sponge-like-precursor was then calcined in airat 700° C., 900° C. and 1000° C. for 8 h in order to decompose theorganic remnants, rendering a black powder as the final product.

Material Characterization

X-ray diffraction (XRD) patterns of samples synthesized in this examplewere collected using a Philips X'Pert PRO ALPHA1 of Panalytical B.V.diffractometer with Cu K_(α1) monochromatic radiation (λ=1.54056 Å) asdescribed in Example 1.

Fullprof Software was employed to carry out structural refinements fromconventional XRD patterns using the Rietveld method again as describedin Example 1.

Scanning electron microscopy (SEM) was carried out as described inExample 1.

High resolution transmission electron microscopy (HRTEM) analysis of theLCFCr powders was performed as described in Example 1. The experimentalHRTEM images were also compared to simulated images using MacTempassoftware. These computations were performed using information from thestructural parameters, obtained from the Rietveld refinement, themicroscope parameters, such as microscope operating voltage (300 kV) andspherical aberration coefficient (0.6 mm), and specimen parameters, suchas zone axis and thickness. The defocus and sample thickness parameterswere optimized by assessing the agreement between model and data.

Microwave-Assisted Synthesis of LCFCr Powders: X-Ray Diffraction andRietveld Refinement

FIG. 9A shows the XRD patterns of the LCFCr powders synthesized by thecombustion method (Method 1), as well as by microwave-assistedcombustion (Method 2), and microwave-assisted sol-gel synthesis (Method3). The diffraction patterns show that a pure crystalline phase isobtained for all three synthesis methods Importantly, the temperatureused did not exceed 1000° C., and without the use of microwave methods,the normal temperature that would have been needed to achieve the sameresult is 1200° C.

FIG. 9B shows the XRD patterns for the material synthesized by themicrowave-combustion method (Method 2) and calcined at three differenttemperatures. It can be seen that, at 700° C., the phase is alreadyforming and at 900° C., the crystalline phase for LCFC has formed. FIG.9C shows the XRD patterns for the material synthesized by themicrowave-assisted sol-gel (Method 3) and calcined at the sametemperatures. It can be seen that, at 700° C. and 900° C., the desiredphase is already forming and similar to Method 2, at 1000° C., thedesired product is present in the pure form.

FIGS. 10A and 10B show the Rietveld refinement fits, respectively, forthe LCFCr samples produced by microwave-combustion method (Method 2,FIG. 9B) and synthesized by the microwave-assisted sol-gel synthesis(Method 3, FIG. 9C). The Rieltveld refinement for the LCFCr powderssynthesized by the regular combustion method (Method 1) is discussed inExample 1. A distorted perovskite structure with an orthorhombicsymmetry (S.G. Pnma, #62) was confirmed for both samples. The unit cellvectors can be represented by √2a_(p)×2a_(p)×√2a_(p), where a_(p) refersto the simple cubic perovskite cell. The results obtained for bothsamples concerning the cells parameters and the atomic positions aresummarized in Table 3.

Microstructural Analysis of LCFCr Powders Synthesized UsingMicrowave-Assisted Methods.

Scanning (SEM) and Transmission Electron Microscopy (TEM)

FIGS. 11A and B show the SEM images of LCFCr powders formed,respectively, using microwave-assisted combustion synthesis at 900° C.(Method 2, FIG. 11A) and microwave-assisted sol-gel synthesis and(Method 3, FIG. 11B). As can be seen, in both cases, the material has aporous morphology, which makes it a good candidate as an electrodematerial. A sponge-like porous morphology can be observed for thepowders formed using Method 2 (FIG. 11A) which is the typical morphologyfound after combustion processes. The sponge-like porous morphology fromMethod 2 is quite different from the morphology obtained using thesol-gel method (Method 3, FIG. 11B) which consists of quite homogeneousagglomerated particles (approximate size 400 nm).

TABLE 3 Structural parameters for LCFCr obtained from Rietveld refinedXRD data. LCFCr Conventional MW - MW - combustion combustion Sol-gel(method 1) (method 2) (method 3) a (Å) 5.4550 (2)  5.4615 (8)  5.4476(4)  b (Å) 7.7128 (1)  7.7470 (7)  7.7194 (2)  c (Å) 5.4552 (2)  5.4619(7)  5.4504 (4)  La/Ca position 4c: X 0.0145 (6)  0.01959 (7)  0.0151(6)  Z −0.003 (3)  −0.003 (1)  −0.0062 (7)  Occ (La/Ca) 0.30 (1)/0.70(1) 0.30 (1)/0.70 (1) 0.30 (1)/0.70 (1) U * 100 (Å²)  0.40 (3)  0.44 (4) 0.52 (2) Fe/Cr position 4b: Occ (Fe/Cr) 0.70 (1)/0.30 (1) 0.70 (1)/0.30(1) 0.70 (1)/0.30 (1) U * 100 (Å²)  0.35 (2)  0.32 (3)  0.43 (2) O (1)position 4c: X 0.502 (2) 0.503 (4) 0.501 (3) Z 0.105 (2) 0.106 (4) 0.106(4) Occ  1.00 (1)  1.00 (1)  1.00 (1) U * 100 (Å²)  0.44 (2)  0.27 (3) 0.41 (5) O (2) position 8d: X 0.297 (4) 0.256 (2) 0.257 (3) Y 0.003 (4)0.005 (3) 0.005 (2) Z −0.254 (3)  −0.31 (2) −0.30 (3) Occ  1.00 (1) 1.00 (1)  1.00 (1) U * 100 (Å²)  0.33 (2)  0.27 (3)  0.41 (5) χ² 1.251.58 1.74 R_(wp)/R_(exp) 4.88/4.37 2.42/1.96 2.65/2.01 (%/%) R_(Bragg)7.30 3.83 5.11 S.G. Pnma: 4c (x ¼ z), 4b (0 0 ½), 8d (xyz)

Further characterization of the material obtained by microwave-assistedsol-gel synthesis (Method 3) at 1000° C. and microwave-assistedcombustion synthesis at 900° C. (Method 2) was performed. Table 4 givesthe elemental analysis of the materials, obtained from the regions inthe squares in FIGS. 11A and 11B. Table 4 shows the atomic percentage ofeach component in the catalysts. The second column of resultscorresponds to the microwave-assisted combustion synthesis (Method 2)and the third column corresponds to microwave-assisted sol-gel synthesis(Method 3). The atomic percentage observed by EDX is comparable to thetheoretical values, based on the expected stoichiometry ofLa_(0.3)Ca_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ). Table 5 provides the specificsurface areas of LCFCr powders formed by Method 1 and Method 2.

TABLE 4 EDX-determined composition (atomic %) of LCFCr powders, formedby microwave-assisted combustion (Method 2) and microwave-assistedsol-gel synthesis (Method 3) approaches Atomic % composition ofLa_(0.3)Ca_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ) MW & comb MW & sol-gelTheoretical La 16 ± 0.5 16 ± 0.5 15 Ca 35 ± 0.5 34 ± 0.5 35 Cr 15 ± 0.515 ± 0.5 15 Fe 34 ± 0.5 36 ± 0.5 35

TABLE 5 Specific surface areas of LCFCr powders formed by regularcombustion (Method 1) and Microwave-assisted combustion (Method 2)Sample S_(BET) (m²g⁻¹) Regular Combustion (Method 1) 0.89Microwave-assisted Combustion (Method 2) 10.4

Transmission Electron Microscopy analysis was also performed on theLCFCr powder obtained using the different synthetic methods describedabove. The cation composition, measured semi-quantitatively by X-rayenergy dispersive spectroscopy in more than ten single crystals is ingood agreement with the theoretical proportions inLa_(0.3)Ca_(0.7)Cr_(0.3)Fe_(0.7)O_(3-δ), confirming the high purity ofthe powder.

In the HTREM images of the crystals prepared by microwave-assistedcombustion synthesis (Method 2) (FIG. 12A) and assisted sol-gelsynthesis (Method 3) (FIG. 12B), nanosized twinned domains are seen. Theappearance of these domains can be associated with the pseudo-cubicnature of these materials. Furthermore, their presence avoids theformation of tetrahedral chains and therefore the formation of undesiredbrownmillerite-type defects [32]. The formation of defects was detectedonly in the sample prepared by sol-gel synthesis, Method 3 (FIG. 12B),showing a periodicity of 1.12 nm. This corresponds to the c axis of theA₃B₃O₈ type structure, which results from the intergrowth of aperovskite ABO₃ and a brownmillerite phase. HTREM images of the crystalsprepared by the regular combustion method (Method 1) are shown in FIG. 3[see also reference 54]. It is worth noting that the microwave-assistedcombustion synthesis (Method 2), as it involves very fast processes,favors disordered phases (perovskite in the present case).

The mixed ion-electron conducting perovskite,La_(0.3)Ca_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ) (LCFCr), was successfullyprepared by microwave-assisted combustion (Method 2) andmicrowave-assisted sol-gel synthesis (Method 3). The desired product wasobtained in crystalline form in only 7 hrs (vs. 13 hrs) and thesynthesis temperature was roughly 300° C. lower than what was found tobe required for conventional solid-state combustion synthesis. The useof microwave has enhanced the rate of formation of the LCFCr powder byseveral orders of magnitude, and also increased the specific surfacearea from 0.89 to 10.4 m² g⁻¹. These results indicate that microwavesynthesis can be used in the preparation of the perovskite materialsused in fuel cells as described herein. Further, the partialsubstitution of Ca for Sr, as described in formula I herein is believedto promote oxygen-vacancy disordering and thus stabilize the perovskitephase vs. the brownmillerite phase. In the HRTEM work herein, theformation of brownmillerite-type defects was detected only in the sampleprepared by sol-gel synthesis (Method 3). In addition, the calcinationtemperature for microwave-assisted combustion (Method 2) was 900° C. vs.1000° C. for microwave-assisted sol-gel synthesis (Method 3). Based onthe lower calcination temperature and the absence of brownmillerite-typedefects, microwave-assisted combustion (Method 2) is a more preferredmethod for synthesis of highly active SOFC cathodes composed of theLa_(0.3)Ca_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ) material.

Example 3: Sulfur Tolerance of La_(0.3)M_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ)(M=Sr, Ca) Solid Oxide Fuel Cell Anodes

Although Ni—YSZ cermets are excellent SOFC anodes, largely because oftheir excellent catalytic activity towards fuel oxidation, Ni issusceptible to poisoning at low levels (1-100 ppm) of H₂S exposure atSOFC operating temperatures (700-1000° C.) [55-59]. It has beensuggested that H₂S inhibits the H₂ oxidation reaction (HOR) ratesbecause it readily dissociates to form a surface adsorbed Ni—S layer(S_(ads)) on catalytic sites normally involved in H₂ dissociation andsubsequent oxidation [58], thereby decreasing the performance of theSOFC.

As a result, extensive research has been carried out to develop sulfurtolerant SOFC anode materials based on Ni-free conducting metal oxides,such as perovskites. For example, La_(0.75)Sr_(0.25)Cr_(0.5)Mn_(0.5)O₃(LSCM) has been reported to exhibit a comparable electrochemicalperformance for hydrogen oxidation as seen at Ni—YSZ at 900° C. [60].However, LSCM has been shown to be less sulfur tolerant in fuelscontaining 10% H₂S [61]. Studies by Mukundan et al [62] showed thatLa_(0.4)Sr_(0.6)TiO₃ (LST) is a sulfur tolerant SOFC anode, as it didnot exhibit any form of degradation in a 5000 ppm H₂S+H₂ fuel. Studiesby Haag et al [63] showed that LaSr₂Fe₂CrO_(9-δ)—Gd_(0.1)Ce_(0.9)O_(2-δ)composite anodes, exposed to 22 ppm H₂S, exhibited only a slightdecrease in performance relative to the response in H₂. Also,La_(0.3)Sr_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ), (LSFCr), operated on wet-(50%H₂+CO) containing 10 ppm H₂S, showed only a small drop in cellpotential, indicating very good stability as an anode insulfur-containing fuels [14].

In contrast, other perovskites have been reported to show an enhancementin the rate of hydrogen oxidation in the presence of H₂S, includingLa_(0.7)Sr_(0.3)VO₃ (LSV[64],Sm_(0.95)Ce_(0.05)Fe_(0.97)Ni_(0.03)O_(3-δ) (SCFN) [65] and Y_(0.9)Sr_(0.1)Cr_(0.9)Fe_(0.1)O_(3-δ) (YSCF) [66]. For LSV, the observedenhancement was attributed to the formation of an active SrS phase,replacing an insulating phase (Sr₃V₂O₈) [64], while for SCFN and YSCF,it was suggested that the active phase that forms in the presence of H₂Sis probably FeS [65, 66]

It has been reported that symmetrical SOFCs, based on the LSFCrperovskite, showed tolerance to low ppm sulfur content in the fuelstream, and also exhibited excellent electrochemical activity towards H₂and CO oxidation, and was also an active oxygen reduction reaction (ORR)material [14].

In this example, the performance of LCFCr as a fuel electrode isstudied. LCFCr is examined as a SOFC anode in H₂ and/or CO atmospheres,with or without H₂S, in comparison to the more well studied LSFCr.Electrochemical methods employing both ac and dc techniques were used ina symmetrical SOFC configuration in H₂ and/or CO fuels, with or withoutthe addition of 9 ppm H₂S, all at 800° C. I LCFCr is found to be a verygood anode catalyst in H₂, CO, and H₂+CO fuels, and this mixedconducting perovskite material demonstrates a fully reversible enhancedcatalytic activity when ca. 10 ppm H₂S is added to the H₂ fuel stream.

A glycine-nitrate combustion process was employed to prepare theLa_(0.3)M_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ) (M=Sr, Ca) perovskite powders,using the method described in Example 1. The ash obtained fromcombustion was subsequently pulverized and pre-calcined at 1200° C. for2 h in air (conditions under which single phases are generated). Powderswere ballmilled (high energy planetary ball mill, Pulverisette 5,Fritsch, Germany) in an isopropanol medium at a rotation speed of 300rpm for 2 h using zirconia balls. TheLa_(0.3)M_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ) (M=Sr, Ca) powders were thenscreen printed symmetrically onto both sides of a 275 μm dense YSZelectrolyte coated with a porous, ca. 10 micron thick SDC buffer layer(Fuel Cell Materials), followed by firing at 1100° C. for 2 h. Au paste(C 5729, Heraeus Inc. Germany) was painted on theLa_(0.3)M_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ) (M=Sr, Ca) layers on both sidesof the pellet to serve as the current collector and Pt wires were usedas the electrical leads.

The cells were fixed in a FCSH-V3 cell holder (MaterialsMate, Italy) forthe purpose of determining their electrochemical properties. A glasssealant (Type 613, Aremco Products, USA) was used to isolate the fueland O₂ sides from each other. The total flow rates were 25 ml/min and 40ml/min at the fuel and O₂ electrodes, respectively. The performance ofthe two electrode cells was evaluated using a four-probe method at 800°C. Impedance spectra were collected under both open circuit andpolarized conditions using a 50 mV perturbation in the frequency rangeof 0.01 Hz to 60 kHz using a Solartron 1287/1255potentiostat/galvanostat/impedance analyzer.

Performance of La_(0.3)M_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ) (M=Sr, Ca) Anodesin Wet 30% H₂/N₂ at 800° C.

A first stage comparison of the performance of LCFCr and LSFCr wascarried out using impedance (EIS) and potentiodymnamic studies. It canbe seen from the Nyquist plot in FIG. 13 that a polarization resistanceof 1.06 and 1.5 Ω·cm₂ was obtained for LCFCr and LSFCr in 30% humidifiedH₂ at 800° C., respectively. The performance of LCFCr is slightly betterthan LSFCr. This is consistent with preliminary electronic conductivityanalysis in H₂ at 800° C. for LCFCr which gave a value of 0.6 S/cm,which is a little higher than the 0.2 S/cm value obtained for LSFCr[14]. Despite this, the EIS response is quite similar for the twomaterials, with two time constants (R/CPE) seen for both LSFCr andLCFCr. From the Bode plot shown in the inset of FIG. 13, the dominantsummit frequencies are seen to be ca. 100 Hz and ca. 1 Hz.

To determine which of the processes arises from the air (cathode) vs.fuel (anode) electrode, the cathode in the LCFCr cell was fed witheither air or pure oxygen. From FIG. 14A, it is clear that the highfrequency (100 Hz) arc can be attributed to the cathode, since in pureO₂, the high frequency resistance (R_(HF)) decreased from 0.37 to 0.28Ω·cm₂, while the low frequency resistance (R_(LF)) remained unchanged,as shown in Table 6. The resistance values were obtained by fitting theNyquist plot in FIG. 14A to the Rs (R_(HF)/CPE_(HF))(R_(LF)/CPE_(LF))equivalent circuit model (FIG. 14B). Rs is the series resistance, Rp isthe polarization resistance (the sum of all of the parallelresistances), and (R_(HF)/CPE_(HF)) and (R_(LF)/CPE_(LF)) are the timeconstants at high (100 Hz) and low (0.5 Hz) frequencies, respectively.Therefore, the low frequency arc (R_(LF)/CPE_(LF)) is predominantly dueto contributions from the fuel electrode (anode) while the highfrequency arc (R_(HF)/CPE_(HF)) arises from the air electrode (cathode),consistent with what has been reported in the literature for other mixedconducting perovskite systems, including LSFCr (10).

TABLE 6 Circuit element values* obtained by fitting the results of FIG.14A** to the R_(S)(R_(HF)/CPE_(HF))R_(LF)/CPE_(LF)) equivalent circuitmodel Cathode Gas R_(S) (Ω · cm²) R_(HF) (Ω · cm²) R_(LF) (Ω · cm²)Rp^(#) (Ω · cm²) Air 1.07 0.37 0.68 1.05 O₂ 1.06 0.28 0.63 0.91 *R_(HF)and R_(LF) obtained form the high (ca. 100 HZ) and low (ca. 1 Hz)frequency arcs, respectively. **Symmetrical fuel cell based on LCFCrelectrodes, operated at 800° C. in wt 30% H₂/N₂ gas mixtures at the fuelelectrode and with air or O₂ exposure at the O₂ electrode. ^(#)Rp =RHF + RLFDC experiments were also carried out to evaluate the performance ofLa_(0.3)M_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ) (M=Sr, Ca). FIG. 15 shows theperformance plots of LCFCr and LSFCr cells operated on 30% humidified H₂at 800° C. LCFCr shows a maximum current and power density of 270 mA/cm²and 142 mW/cm², respectively, while the analogous values for LSFCr are255 mA/cm² and 134 mW/cm², consistent with the EIS data in FIG. 13. Theperformance of our La_(0.3)M_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ) (M=Sr, Ca)based cells is comparable to that of other symmetrical cells (based onperovskite electrodes) reported in the literature, such asLa₄Sr₈Ti_(12-x)Fe_(x)O_(38-δ) (LSTF), which showed power densities of90-100 mW/cm² at 950° C. in humidified H₂ (16).Performance of LCFCr in Other Fuel Mixtures at 800° C.

The performance of the LCFCr electrode was also examined in CO andsyngas (H₂+CO) atmospheres. FIG. 16 shows that the polarizationresistance of the cell is 0.95 Ω·cm² in H₂, which is slightly smallerthan both the Rp values obtained in CO (1.11 Ω·cm²) and CO+H₂ (1.00Ω·cm²) atmospheres. This indicates that the material is only a somewhatbetter catalyst for H₂ oxidation than CO oxidation. This shows thatLCFCr is a useful SOFC anode material that can be employed in a range offuels, giving a very good performance in all cases. Also, from theNyquist plot, it is seen that it is the low frequency arc(R_(LF)/CPE_(LF)) that is changing with changing fuel environments,again confirming that the low frequency arc is associated primarily withthe anode, as indicated above. The performance plot of the cell in thesethree gases is shown in FIG. 17. The OCP in the three gases is seen tobe ca. 1.06 V, which is very close to the theoretical value, indicatingthat the anode and cathode compartments are well sealed and that thereis no gas leakage. As stated earlier, there is not much difference inthe activity of the LCFCr material in H₂, CO and H₂+CO atmospheres, andthis is supported here by the dc measurements. The maximum power densityobtained is between 140 to 150 mW/cm² in all of the environments, whilethe maximum current density is in the range of 250-270 mA/cm², all at800° C.

Effect of Low Ppm H₂S on Performance of LCFCr in 30% H₂ (Bal Wet N₂) at800° C.

FIG. 18A shows that, when 9 ppm H₂S is added to 30% H₂ under OCPconditions, the polarization resistance decreased slightly, from 1.00 to0.96 Ω·cm₂, translating to a ca. 4% decrease in Rp in the presence ofH₂S. No poisoning/deactivation of the LCFCr at 800° C. is seen. This wasalso seen for LSFCr, although only the results for LCFCr are shown here.In comparison, most of the sulfur-induced performance enhancementbehavior reported for other types of perovskites has usually beenobserved at much higher concentrations of H₂S (1-5%). To betterunderstand these results, the effect of ac polarization on the cell wasinvestigated. FIGS. 18B and 18C show the polarized EIS results for theLCFCr-based cell in H₂, with or without the addition of 9 ppm H₂S at800° C. When the cell was polarized at −100 mV vs. the full cell opencircuit voltage, i.e., at a cell voltage of ca. 0.95 V, (FIG. 18B), Rpdecreased from 0.97 Ω·cm² in H₂ to 0.90 Ω·cm² in the presence of H₂S,while when the anode was polarized at −300 mV (ca. 0.75 V cell voltage),Rp decreased from 0.80 to 0.72. Ω·cm² (FIG. 18C). The plot in FIG. 18Eshows the % Rp change vs. the applied voltage, calculated based on theRp data in FIG. 18D (% Rp change=[(Rp_(H2)−Rp_(H2S))]×100/(Rp_(H2))). Itcan be seen that Rp decreased by 4% at the cell OCP and by 7% and 11% inthe presence of H₂S when the cell was polarized at −100 and −300 mV vs.the full cell open circuit voltage, respectively. This indicates thatthe enhancement of the performance of LCFCr in H₂ in the presence of H₂Simproves with polarization at 800° C.

As stated earlier, some ferrite-based perovskites, such asSm_(0.95)Ce_(0.05)Fe_(0.97)Ni_(0.03)O_(3-δ) [65] and Y_(0.9)Sr_(0.1)Cr_(0.9)Fe_(0.1)O_(3-δ) [66], have been reported to showenhanced H₂ oxidation activity or electrochemical oxidation of H₂S onlyin high concentrations of H₂S (1-5%), due to the formation of sulfidespecies (e.g., FeS) at 600-800° C. On the other hand, probably under thepresent testing conditions, some type of adsorbed surface sulfidespecies (possibly FeS) is being formed, even when H₂S is present at ppmlevels at 800° C. However, detailed surface characterization studies arerequired to confirm the presence of FeS in these experiments.

To further study the performance of the LCFCr electrode towards H₂oxidation in the presence or absence of 9 ppm H₂S, potentiostaticstudies were also carried out at the −100 and −300 mV vs. OCP cellpolarization, respectively. FIG. 19A shows the results of polarizationat −100 mV vs. the full cell voltage at open circuit for 9 h with andwithout H₂S. As can be seen, upon the addition of 9 ppm H₂S to the H₂fuel, the current density increased from about 95 mA/cm² to 98 mA/cm²,giving a 2.1% improvement in performance. After 4 h of removal of H₂S,the current density decreased to about 95 mA/cm², similar to the valueobserved before H₂S exposure. This shows that the enhancement is onlyobserved in the presence of H₂S and that the cell fully recovers in theabsence of H₂S, suggesting that no bulk sulfide phase forms, but ratheronly a surface species is generated. This behavior is also seen at −300mV vs. the full cell voltage at open circuit (FIG. 19B), where the cellimproved by 4.3% in the presence of H₂S and fully recovered when the H₂Swas removed.

This example has focused on the development of mixed conductingperovskite oxides for use at both electrodes in reversible solid oxidefuel cells (RSOFCs). In this example, the performance of LCFCr, incomparison with LSFCr, has been investigated in a range of fuelenvironments, with and without ppm levels of H₂S, all at 800° C. Thesymmetrical fuel cells were constructed by screen-printingLa_(0.3)M_(0.7)Fe_(0.7)Cr_(0.3)O_(3-δ) (M=Sr, Ca) on a Yttria-stabilizedzirconia (YSZ) electrolyte covered by a thin Samaria-doped ceria (SDC)buffer layer, and then tested using both impedance and potentiostatictechniques. LCFCr is an equally good fuel electrode (anode) and O₂electrode (cathode) as LSFCr, exhibiting very good electrochemicalperformance in H₂, CO and syngas (H₂+CO) atmospheres, givingpolarization resistance of 0.95 Ω·cm² in wet 30% H₂ and 1.00 Ω·cm² and1.11 Ω·cm² in wet 15% H₂:15% CO and 30% CO atmospheres, respectively.The maximum power density obtained using these gases were between 140 to150 mW/cm², while the maximum current density was in the range of250-270 mA/cm². The LCFCr and LSFCr anodes were also evaluated in thepresence of 9 ppm H₂S, showing a small, but reproducible and reversible,decrease in polarization resistance (Rp). Chronoamperometric studies atcell polarizations of −100 and −300 mV vs. the full cell voltage at opencircuit showed a ca. 2-4% increase in current density in the presence of9 ppm H₂S+30% H₂, with the cell recovering fully when H₂S was removed.This is noteworthy and indicates that some type of adsorbed surfacesulfide species (possibly FeS) is being formed in the presence of lowppm H₂S at 800° C., leading to the observe enhancement in hydrogenoxidation activity.

Example 4: Fabrication of Solid Oxide Fuel Cells by Microwave MethodsHalf-Cell Construction and Testing

In this example, six different types of cells were prepared and tested,with cells composed of MW-combusted electrodes, sintered using aconventional furnace, used as blanks (Cell 1, Table 1). LCFCr powderswere prepared using microwave (MW)-assisted combustion, starting withthe appropriate nitrate precursors. Details of the synthetic procedureare in Example 2. The LCFCr powders were milled (high energy planetaryball mill, Pulverisette 5, Fritsch, Germany) in an isopropanol medium ata rotation speed of 300 rpm for 2 h using zirconia balls. The ca. 30 μmthick LCFCr electrodes were then screen-printed symmetrically (over anarea of 0.5 cm²) onto both sides of commercial gadolinium-doped ceria(GDC) electrolytes (Fuel Cell Materials, 1 mm or 250 microns thick),forming a symmetrical LCFCr/GDC/LCFCr cell. Au paste (C 5729, HeraeusInc., Germany) was painted on each of the electrode layers to serve asthe current collectors.

Cell 1 was constructed from MW-combusted LCFCr electrodes but was thensintered using a conventional furnace and used as the blank (Table 7).In order to clearly compare the effect of the conventional furnacesintering time and the temperature, Cell 1 was heated at 1100° C. for 2hours with a ramp rate of 5° C./min, which is our typical electrodesintering condition 26-28. Considering the ramp time, sintering time,and cooling time, the overall electrode sintering time for thepreparation of Cell 1, using a conventional furnace, was 10 hours (Table7).

Cells 2-6 (Table 7) were prepared without the use of a standard furnace,and were instead constructed using the MW-prepared LCFCr electrodepowders, followed by MW sintering of the complete cells using aMilestone MultiFAST-6 microwave instrument. Due to its silicon carbidecrucibles, this microwave fusion system can reach temperatures of ca.1000° C. in only a few minutes. Four different MW sintering temperatureswere used (600, 700, 850 and 900° C.) in order to optimize the electrodepreparation conditions. Although the MW-heated cell sinteringtemperature was measured with an infrared sensor directly in the siliconcarbide crucible, there are inherent difficulties in precisely measuringthe ‘true’ sample temperature during MW exposure and thus thetemperatures are reported only to a few significant figures.

In all cases (Cells 2-6), the sintering time at the desired temperaturewas 20 minutes. However, in some cases (not all cells were heated at thesame rate), the ramp time had to be adjusted to reach the desiredtemperature, i.e., the ramping time was 30 minutes for cells 2, 3 and 5and 45 minutes for cells 4 and 6.

Electrochemical measurements to evaluate cell performance were performedusing 2 electrode methods in air. Electrochemical impedance spectra(EIS) were collected under open circuit conditions at 800° C., using anac amplitude of 50 mV in the frequency range of 0.01 to 65 kHz and usinga Solartron 1287/1255 potentiostat/galvanostat/impedance analyzer. Zviewsoftware was used to fit and analyze the impedance data.

TABLE 7 Cell fabrication description and total polarization resistance.Powder Cell Sintering synthesis sintering Electrolyte temperature Rp*Cell method heat source thickness and time Total time (Ω · cm²) 1 MWFurnace 1 mm 1100° C., 2 h 10 h 0.50 combustion 2 MW MW 1 mm 700° C., 20mins 1 h 20 mins 0.40 combustion 3 MW MW 1 mm 900° C., 20 mins 1 h 20mins 0.25 combustion 4 MW MW 250 μm 850° C., 20 mins 1 h 20 mins 0.62combustion 5 MW MW 250 μm 850° C., 20 mins 1 h 35 mins 0.48 combustion 6MW MW 250 μm 600° C., 20 mins 1 h 35 mins 0.60 combustion *Two electrodeLCFCr/GDC/LCFCr half-cell configuration, 800° C., pO₂ = 0.21 atm, 50ml/min flow rate.

All samples were examined by scanning electron microscopy (SEM) asdescribed in Example 1.

FIG. 20A shows the SEM image of an as-prepared LCFCr electrode in Cell 1(MW prepared electrodes, followed by furnace sintering of the cell),revealing an average particle size of ca. 1 μm. It is worth noting thenon-homogeneous character of the LCFCr layers, possibly due to the factthat the MW-prepared LCFCr powders were calcined at a lower temperature[84] (900° C.) than the electrode sintering temperature, thus theparticles may have sintered when they were exposed to highertemperatures.

FIG. 20B shows the change in morphology of Cell 1 after ca. 100 hours ofelectrochemical testing in air at 800° C. Now, an average LCFCr particlesize of ca. 2 μm is obtained and a more homogeneous morphology of theLCFCr electrode is observed overall. This may be due to the fact that,during the ca. 100 hours of operation at 800° C., the LCFCr particleshave sintered further.

FIGS. 21A and 21B show SEM images of the LCFCr electrode in Cell 2 (MWprepared powders followed by MW sintering of the cell). FIG. 21A showsthat a much smaller average particle size of ca. 50 nm is observed whencompared to the cells sintered using conventional furnace sintering(FIG. 20A), and a very homogenous morphology is also seen. In the caseof the electrode after electrochemical testing (FIG. 21B), the LCFCrelectrode presents a very similar morphology to the as-prepared cell(FIG. 21A). In fact, after ca. 100 hours of operation at 800° C. in air(FIG. 21B), the MW prepared LCFCr electrode in the MW sintered cellshows an unchanged average particle size of ca. 50 nm after celloperation This set of cells were MW sintered at 900° C., 300° C. lowerthan the conventionally sintered cells shown in FIG. 20A, which likelyexplains why, in this case, the LCFCr electrode has a much smallerparticle size overall.

The electrochemical impedance spectra of the cells fabricated with a 1mm thick GDC electrolyte (Cells 1-3) are shown in FIG. 22A. Thefurnace-sintered cell (Cell 1) and the MW sintered cells, sintered at700° C. (Cell 2) and 900° C. (Cell 3), were then evaluated for theirperformance in a half-cell configuration in air at 800° C. (FIG. 22B).The MW sintered cells show a better performance, overall, with thepolarization resistance (Rp) decreasing from 0.50 (Cell 1) to 0.40 (Cell2) and 0.25 Ωcm² (Cell 3). To further study the effect of the MW cellsintering temperature, Cells 2 and 3 were tested at 700 and 900° C.,keeping the sintering time as well as the ramping time constant. FIG.22A shows that the cell that was MW sintered at 900° C. vs. at 700° C.has a better performance, giving Rp values of 0.25 and 0.40 Ωcm2,respectively. As shown in FIG. 22A, the ohmic resistance values alsodecrease, showing the same trend as the polarization resistance (Cell3<Cell 2<Cell 1). Cell 3 (MW sintered at 900° C.) exhibits the lowest Rpas well a slightly lower ohmic resistance of ca. 0.59 Ωcm²

FIG. 23 shows the electrochemical impedance spectra of samplesfabricated with a thinner (250 μm) GDC electrolyte (Cells 4-6). It wasintended to sinter these cells at 900° C. and 700° C., respectively, inorder to compare their performance to what is seen in FIG. 22A. However,the cells with the thicker GDC electrolytes (Cells 2-3) reached thedesired temperature more rapidly than the cells with the thinner GDCelectrolyte (Cells 4-6). All of the samples were sintered using the sameMW frequency and power (2.45 GHz and 1500 W) and yet, for Cells 4-6, thedesired temperatures of 900° C. and 700° C. were not reached. As aresult, Cell 4 reached a sintering temperature of 850° C. using the sameramping times and sintering times as in the case of Cells 2 and 3. Inorder to attempt to reach the desired temperatures, the ramping timeswere increased from 30 to 45 minutes in the construction of Cells 5 and6.

As shown in FIG. 23, the polarization resistance for Cells 4 and 5, bothMW-sintered at 850° C., decreased from 0.62 to 0.48 Ωcm² when a longerramping time was used, while the polarization resistance of the cellsintered at 600° C. with a longer ramping time (Cell 6) was 0.60 Ωcm².These results indicate that the thickness of the electrolyte affects theMW sinterability of the cell and thus enhances the bonding of theelectrodes onto the electrolyte surface. Also, it appears that theramping time to the desired MW sintering temperature plays an importantrole in the sintering process.

This example provides an effective technique foranode-electrolyte-cathode co-sintering in one simple step for SOFCs.Cells sintered with MW methods (Cells 2-6) had an average particle size(ca. 50 nm) significantly smaller that the cells sintered withconventional furnace (ca. 100 μm). The GDC electrolyte thickens has tobe taken into account in achieving desired sintering temperature by MWmethods, thicker (1 mm) electrolytes successfully reached the desiredtemperatures, while in the case of the thinner GDC electrolytes (250μm), the ramping times had to be increased (from 30 to 45 minutes) toreach the desired temperatures.

Overall, the 1 mm GDC electrolyte cells types (sample 3), which were MWsintered at 900° C. for 20 min, reaching the desired temperature in 30minutes showed the best performance. These types of cells were sinteredin a total time of 1 hour and 20 minutes (vs 10 hours in conventionalfurnace) and showed a polarization resistance of 0.25 Ωcm² perelectrode, which is in the range of most of the state of the artcathodes prepared by conventional solid state methods and sintered byconventional furnace heating. LCFCr electrodes can be co-sintered in onesingle step onto the GDC electrolyte in ca. 5 times less time than witha conventional furnace.

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The invention claimed is:
 1. An electrode material having the formula:La_(w)M_(x)Fe_(y)Cr_(z)O_(3-δ) where: M is a mixture of Ca and Sr wherethe molar ratio of Ca to Sr ranges from 1:1 to 100:1; w is 0.2 to 0.4; xis 0.6 to 0.8; y is 0.6 to 0.8; z is 0.2 to 0.4; and δ represents oxygendeficiency.
 2. The electrode material of claim 1 wherein w is 0.27 to0.33; x is 0.67 to 0.73; y is 0.67 to 0.73; and z is 0.27 to 0.33. 3.The electrode material of claim 1 wherein w is 0.29 to 0.31; x is 0.69to 0.71; y is 0.69 to 0.1; and z is 0.29 to 0.31.
 4. The electrodematerial of claim 1 wherein w is 0.3; x is 0.7; y is 0.7; and z is 0.3.5. The electrode material of claim 1, wherein the molar ratio of Ca toSr is 1:1.
 6. The electrode material of claim 1, wherein the molar ratioof Ca to Sr is 10:1.
 7. An electrode for a solid oxide fuel cell whichcomprises the electrode material of claim
 1. 8. A fuel electrode whichcomprises the electrode material of claim
 1. 9. An air or oxygenelectrode which comprises the electrode material of claim
 1. 10. Theelectrode material of claim 1, which is prepared by microwave-assistedcombustion, microwave-assisted co-precipitation or a microwave-assistedsol-gel method.
 11. A solid oxide fuel cell having an electrode whichcomprises the electrode material of claim
 1. 12. A reversible solidoxide fuel cell having an electrode which comprises the electrodematerial of claim
 1. 13. A method for selectively generating electricityor employing electricity to generate a fuel which comprises selectivelyoperating a reversible solid oxide fuel cell of claim 10 to generateelectricity or to generate a fuel.
 14. The method of claim 13, whereinthe solid oxide fuel cell or reversible solid oxide fuel cell isoperated in the presence of a fuel containing hydrogen sulfide.
 15. Themethod of claim 13 operated at a temperature in the range of 600-850° C.16. A method for sintering a solid oxide fuel cell (SOFC) or areversible solid oxide fuel cell (RSOFC) which comprises the steps of:providing a dense solid electrolyte having a first and second oppositesurfaces, applying a slurry or paste of a first and second electrodematerial to the first and second opposite surfaces of the solidelectrolyte to form a first and second layer; irradiating the resultingsolid electrolyte with first and second layers with microwave radiationto reach a target temperature ranging from 600 to 900° C. over a ramptime ranging from 20 to 60 minutes and thereafter maintainingirradiation at the target temperature for 15 to 30 minutes, wherein theelectrode material is an electrode material of claim 1.